Method and apparatus for robust two-stage OFDM channel estimation

Abstract

Methods, apparatuses, and systems for improved channel estimation in an Orthogonal Frequency Division Multiplexing (OFDM) system are discussed. In one example discussed herein, joint two-dimensional Minimum Mean-Square Error (2D-MMSE) channel estimation is performed on any Resource Element (REs) containing a reference signal in a Resource Block (RB), one-dimensional Minimum Mean-Square Error (1D-MMSE) channel estimation is performed in the frequency domain on each OFDM symbol in the RB, and then channel estimation is performed in the time domain on each frequency sub-carrier in the RB. In another example discussed herein, Power Delay Profiles (PDPs) and / or frequency correlations are calculated using minimax optimization and then stored in a Look-Up Table (LUT) indexed by estimated Signal to Noise Ratio (SNR) and the estimated maximum delay spread. A portable device could use such an LUT in MMSE calculations.

Description

PRIORITY[0001]The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61 / 939,649, entitled “Method and Apparatus for Robust Two-Stage OFDM Channel Estimations,” which was filed on Feb. 13, 2014, the contents of which are incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates generally to channel estimation, and more particularly to more robust and better performing channel estimation in an Orthogonal Frequency Division Multiplexing (OFDM) system.[0004]2. Description of the Related Art[0005]Modern cellular systems such as the 4th generation Long Term Evolution (LTE) and WiMax networks depend on coherent detection for data communications to achieve high performance, and coherent detection requires Channel State Information (CSI) to be implemented. For OFDM systems, pilot-aided channel estimation is an effective way to obtain CSI at the receiver side.[0006]In one example ...

AI Technical Summary

Benefits of technology

The present invention provides a method for a receiver to estimate the conditions of a channel through which it receives OFDM signals. The method involves performing joint 2D and 1D Minimum Mean-Square Error (MMSE) channel estimation on the received signals. The data used in the joint 2D-MMSE channel estimation is from the time-domain and frequency-domain, while the data used in the 1D-MMSE channel estimation is from the frequency-direction. The method also includes finding at least one Power Delay Profile (PDP) or frequency correlation in a Look-Up Table (LUT) and calculating an MMSE weight using minimax optimization. This method provides greater accuracy and lower complexity than other alternatives. A portable device that performs these steps is also provided.

Problems solved by technology

The 2D channel estimation has better performance at the cost of high computational complexity which also leads to large processing delay.

However, when there is a large delay spread and high Signal to Noise Ratio (SNR), there is a considerable performance gap between 2D and 2×1D.

Such channel statistics can be estimated in some cases, such as when the CRS is continuously transmitted in the LTE system, but this necessarily causes extra complexity in the receiver.

However, it is very difficult, and in some cases impossible, to estimate the PDP using the UE-specific DMRS defined in the LTE standard, which has precoding which may change from subframe to subframe and PRB (Physical Resource Block) to PRB.

The differences between the UE-specific reference signal DMRS and the cell-specific reference signal CRS cause some problems unique to DMRS:Unlike wide-band CRS, denoising in the time domain is not a viable approach to reduce the noise level for DMRS channel estimation.Unlike CRS, in which MMSE weights can be derived from estimated channel statistics (i.e., PDP), estimated channel statistics like PDP are not suitable for DMRS because DMRS precoding may change from PRB to PRB and / or from subframe to subframe.The usually robust uniform distribution PDP model incurs mismatch loss when used for DMRS channel estimation especially at high SNR with large delay spreads.In addition to the knowledge or assumption of PDP, the MMSE estimator also demands the information of equivalent SNR.

However, for DMRS, there may be some power mismatch between CRS and DMRS due to precoding.

Such a mismatch will result in unavoidable performance loss as well.For CRS, different antenna ports are orthogonal in time and frequency and there are no interferences among them.

For DMRS in a Multiple User Multiple Input Multiple Output (MU-MIMO) environment, two antenna ports or more may use non-orthogonal random sequences in generating DMRS sequences, resulting in cross interferences among them.

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